In a typical coal seam gas and oil well 10, a sucker rod string 12 is lowered down the inside of a production pipe 14, as shown in FIG. 1. The production pipe 14 follows an underground coal seam 15. The sucker rod string 12 is driven by a motor 16 at the ground surface 18. The motor 16 rotates or reciprocates the sucker rod string 12 (depending on the type of pump) in order to drive a removal pump 20 mounted at the bottom of the well 10. The oil, water, entrained sand and coal dust particles form an abrasive fluid which gets pumped to the ground surface 18 by passing up the inside of the production pipe 14 within which the sucker rod string 12 which is rotating. Such a sucker rod string 12 is made up by screwing together rod components 22 shown in FIGS. 1 and 2.
Referring to FIG. 2, the rod components 22 are steel bars (which are typically 8 meters long and around 22.5 mm in diameter). The rod components 22 are connected by interposing a coupler 24 (having a female thread) between each rod component 22 (having a male thread). The total length of the sucker rod string 12 down the well 10 may be perhaps 600 metres long. The sucker rod string 12 may rotate at around 300 rotations per minute to drive the bottom-hole pump 20. The well 10 may be deviated from vertical, so that the rod string 12 must rotate within a pipe 14 that is descending thorough the production pipe 14. However, as the pipe production 14 is deviated and the fluid being pumped is highly abrasive, the sucker rod string 12 and the production pipe 14 are rapidly worn. Wear points 21 are shown on the sucker rod string 12 in FIG. 1. This frequently leads to a failure of the well 10 which is costly to repair.
The method which is commonly used to minimise the wear of the rod components and pipes involves the use of rod guides called “centralisers” made from a wear resistant polymer. These centralisers are attached to each rod component and act as bearings and spacers to keep the rotating rod components away from the internal surface of the pipes. There are two popular methods of connecting centralizers to sucker rods. The first method involves manufacturing the centralizers in a factory, transporting the centralizers to a well and then manually connecting the centralizers to the sucker rods using a clip-on mechanism. A prior art centralizer 26 is shown in FIG. 2 on a sucker rod component 22. Another popular method of attaching centralizers to the sucker rod involves attaching a mould to a sucker rod and injecting the mould with plastic. The mould is then removed, leaving a moulded centralizer 26 attached to a sucker rod component 20. Typically around three centralizers are attached to each sucker rod component 22.
However, due to the extremely abrasive nature of the sand and formation coal fines, the prior art centralizers 26 are prone to rapid wearing. This exposes the sucker rod string 12 and production pipe 14 to damage and frequently results in the failure of the well. To exacerbate the problem, the operator has no method of detecting that the abrasion damage has reached critical levels, so that by the time the operator finds out the well is not operating the internal components of the sucker rod string and production pipe may already be destroyed.
It is an object of the invention to provide a centralizer Which is longer wearing and provides a signal for the onset of wear before damage occurs to the sucker rod and production pipe.